Spending other people's money is such fun.
If you don't already have a converter, Liz, I would think twice before getting one in your current circumstances.
As Ian said, not all lenses will work with converters, particularly Canon equipment. You do lose a little bit of light and some loss of sharpness although that shouldn't be too much of a problem with a Canon L lens and a 1.4x converter.
They work with the Canon 70-200 range, the 100-400 and a few prime (fixed size) lenses but not with most Canon lenses. Some third party converters will work with other lenses but quality can be variable.
However, a converter will add a bit of extra length and weight; and when you add together the price of lens plus converter there won't be a lot of difference between a 70-200 plus converter and the 70-300 L lens.
Now macro. What exactly do you want to photograph? For flowers or larger/very approachable insects etc, a 100 mm lens will suffice but I would recommend going to 150 mm for any serious work on small insects. The Sigma 150 is a popular choice amongst entomologists.
However, another option is an extension tube which fits between your lens and the camera and allows you to focus a little closer. This would work reasonably well with either of the suggested zooms for flowers/butterflies, etc. But I would recommend a proper macro lens for anything smaller.
A 25 mm tube would work OK and cost a little over £100 (in the UK) however they are currently in short supply. A good tripod will also be necessary.
Which just leaves a shorter lens. I use a Canon 24-105 L IS which suits me, although some people like something a little smaller. My friend with the 70-300 L also uses something around 15-85 mm approx (I forget the exact size) which is a Canon lens with IS but not an 'L' lens. However it seems to produce excellent results and is slightly cheaper.